Problem: $\dfrac{9}{10} - \dfrac{7}{4} = {?}$
Solution: $ = {\dfrac{9 \times 2}{10 \times 2}} - {\dfrac{7 \times 5}{4 \times 5}} $ $ = {\dfrac{18}{20}} - {\dfrac{35}{20}} $ $ = \dfrac{{18} - {35}}{20} $ $ = -\dfrac{17}{20}$